The present invention provides processes for activating monolith adsorbents and for partially decomposing and/or removing the binding agents present in the monolith. More particularly, the present invention provides processes for using these monoliths in gas/vapor separation operations.
Cyclic adsorption processes are frequently used to separate the components of a gas mixture. Typically, cyclic adsorption processes are conducted in one or more adsorbent vessels that are packed with a particulate adsorbent material which adsorbs at least one gaseous component of the gas mixture more strongly than it adsorbs at least one other component of the mixture. The adsorption process comprises repeatedly performing a series of steps, the specific steps of the sequence depending upon the particular cyclic adsorption process being carried out.
In any cyclic adsorption process, the adsorbent bed has a finite capacity to adsorb a given gaseous component and, therefore, the adsorbent requires periodic regeneration to restore its adsorption capacity. The procedure followed for regenerating the adsorbent varies according to the process. In VSA processes, the adsorbent is at least partially regenerated by creating vacuum in the adsorption vessel, thereby causing adsorbed component to be desorbed from the adsorbent, whereas in PSA processes, the adsorbent is regenerated at atmospheric pressure. In both VSA and PSA processes, the adsorption step is carried out at a pressure higher than the desorption or regeneration pressure.
A typical VSA process generally comprises of a series of four basic steps that includes (i) pressurization of the bed to the required pressure, (ii) production of the product gas at required purity, (iii) evacuation of the bed to a certain minimum pressure, and (iv) purging the bed with product gas under vacuum conditions. In addition a pressure equalization or bed balance step may also be present. This step basically minimizes vent losses and helps in improving process efficiency. The PSA process is similar but differs in that the bed is depressurized to atmospheric pressure and then purged with product gas at atmospheric pressure.
As mentioned above, the regeneration process includes a purge step during which a regeneration gas stream that is depleted in the component to be desorbed is passed countercurrently through the bed of adsorbent, thereby reducing the partial pressure of adsorbed component in the adsorption vessel which causes additional adsorbed component to be desorbed from the adsorbent. The non-adsorbed gas product may be used to purge the adsorbent beds since this gas is usually quite depleted in the adsorbed component of the feed gas mixture. It often requires a considerable quantity of purge gas to adequately regenerate the adsorbent. For example, it is not unusual to use half of the non-adsorbed product gas produced during the previous production step to restore the adsorbent to the desired extent. The purge gas requirement in both VSA and PSA processes are optimization parameters and depend on the specific design of the plant and within the purview of one having ordinary skill in the art of gas separation.
Many process improvements have been made to this simple cycle design in order to reduce power consumption, improve product recovery and purity, and increase product flow rate. These have included multi-bed processes, single-column rapid pressure swing adsorption and, more recently, piston-driven rapid pressure swing adsorption and radial flow rapid pressure swing adsorption. The trend toward shorter cycle times is driven by the desire to design more compact processes with lower capital costs and lower power requirements. The objective has been to develop an adsorbent configuration that demonstrates a low pressure drop, a fast pressurization time and an ability to produce the required purity of oxygen.
Honeycomb structured monoliths, which are normally made by high temperature treatment of a mixture of binders, additives and catalyst or adsorbent materials are suitable for fast cycle sorption processes. These monoliths, either in the form of one single block or in the form of extrudates with multiple random channels, exhibit unique features of low pressure drop, good mechanical properties and freedom from attrition and fluidization problems of conventional catalysts and adsorbents. These types of monoliths have historically been employed as catalyst supports in automobile catalytic converters, catalytic combustion, electrochemical reactors and biochemical reactors. These monoliths however have very low loadings of active catalyst or adsorbent and not all of the adsorbent or catalyst material is accessible to the gas molecules passing through them.
Monoliths, however, that are made from paper like sheets containing polymeric fibers as described in U.S. Pat. Nos. 5,660,048; 5,660,221; 5,685,897; and 5,580,369, exhibit very high loadings of adsorbent material. Active adsorbent materials such as zeolites, carbon molecular sieve (CMS), alumina and other porous adsorbent materials can be embedded in the paper during the manufacturing process. In order to bind adsorbent particles with fibers and to have uniform distribution of adsorbent particles, many ingredients and additives may also be added into the slurry during the sheet manufacturing. Normally, the non-woven-fabric sheet (paper), which will be shaped into the monolith in later stages, comprises fibers such as polyaramids, one or more binders such as acrylic latex, a flocculating agent and active adsorbent materials.
The binder is added to the slurry to bind the adsorbent particles to the fibers. Through this process, adsorbent/catalyst particles tend also to be encapsulated by the polymeric binder material. The adsorbent containing monoliths or sheets need to be activated at high temperatures to desorb water or other sorbed species from their active sites. The temperature at which the activation is undertaken is dependent on the nature of the adsorbent material. As described in U.S. Pat. No. 5,580,369, this activation is typically performed at temperatures below the decomposition temperature of the binder. This degree of activation is sufficient, if the adsorbate molecules can diffuse through the binder layer to reach the adsorbent/catalyst. It has been demonstrated that such an activation below the temperature of binder decomposition works well in dehumidification applications, where the diffusivity or solubility of water through the binder is high and adsorbent does not need higher temperature activation. However, it has been found in the current invention that in certain types of applications such as the adsorption of N2 from air, the binder layer typically provides a kinetic barrier to adsorbate molecules from reaching the adsorbent particles. When this happens, the resulting monolith has poor kinetic performance for gas adsorption and desorption, which results in poor PSA/VSA performance in gas separation processes.
Low temperature activations may also be suitable for adsorbent materials, which do not interact strongly with moisture or other contaminants. However, adsorbents such as zeolites, particularly Li containing zeolites of type X and A require activation temperatures of at least 300xc2x0 C., preferably greater than 400xc2x0 C.
The present invention provides a novel method of activating monolith adsorbent and a binder decomposition such that improved adsorbent properties result.
The present invention provides for processes for removing binding agents from monolith adsorbents. These processes comprise passing a heated regeneration gas stream through the monolith at a temperature sufficient to decompose, at least part of the binder agent. The heated regeneration gas will also activate the active adsorbent materials that are contained within the monolith.
The present invention also provides for an improved process for preparing a monolith adsorbent comprising the steps of forming a slurry comprising water, fiber, binder, adsorbent and flocculating agent; forming paper from said slurry; forming a monolith containing said paper; the improvement comprising passing heated regeneration gas through the monolith, thereby decomposing the binding agent and activating the adsorbents contained therein.
The present invention further provides for a method for separating a first gaseous component from a gas mixture by passing the gas mixture into an adsorption zone containing a monolith adsorbent that has been treated by passing a heated regeneration gas through it thereby separating the first gaseous component from the gas mixture and recovering the adsorbed first gaseous component.
The present invention provides a method for removing more than 50% of the binding agents from a monolith structure comprising passing a regeneration gas stream through the monolith at a temperature which will decompose part of the binding agent, and preferably 70-98% of the binding agent in the monolith.
The monolith structure typically contains an adsorbent which is included in the slurry mixture by which the paper used to manufacture the monolith is produced. The slurry is typically a mixture of water, natural or synthetic fibers, binders, flocculating agents, and additional chemicals which after being dried by gravity drain and/or by application of vacuum, can be formed into sheets. These sheets can then be formed into the appropriate shape for the monolith. Preferably, the monolith adsorbents that are treated by the methods of the present invention are already formed into preferred shapes.
The regeneration gas that is utilized may be an inert gas such as argon and nitrogen or an inert gas containing another gas component, which can oxidize organic species under the activation conditions such as oxygen, nitrous oxide and ozone or other type of gas like hydrogen, preferably nitrogen, air or mixtures thereof. This regeneration gas is heated to a temperature ranging from about 300xc2x0 C. to about 500xc2x0 C. The temperature to which the regeneration gas is heated will be dependent on the type of binder agent employed in forming the sheet, the nature of adsorbent in the monolith and the design of the finished formed monolith adsorbent.
The binding agents that are employed in manufacturing the paper sheet include but are not limited to acrylic latex, starch, polyvinyl alcohols, acrylics, polyurethane, polyethylene glycol, polypropylene glycol, polyacrylic acid, polyamide and polyamine. Other non-polymeric binders having a functionality of a carboxylic acid, aldehyde, amino acid, and amine can be employed. Inorganic binders such as silica and mineral silicates may also be used in manufacturing the paper sheet. When inorganic binders are used, only the organic functionality will be decomposed in addition to other organic chemical additives added during the stock preparation.
The fibers that are employed in manufacturing the sheets can be synthetic or natural and can be organic or inorganic. The synthetic organic polymeric fibers include aromatic polyamides, polyesters, polyvinyl chlorides, nylons, acrylics, polyethylenes, polypropylenes, acrylonitrile homopolymers, copolymers with halogenated monomers, styrene copolymers, and mixtures of polymers (polypropylene with low-density polyethylene, and high density polyethylene with polystyrene). The inorganic fibers include glass or metal fibers and rock wool etc. The natural fibers include wood pulp such as cellulose. Combination of organic and inorganic fibers can also be used. Fibrillated fibers, as described in U.S. Pat. No. 5,685,897, can also be used.
As noted above, the monolith structure when formed may take on any design and shape for use in gas separation processes. The formed monolith structure may include as adsorbents of zeolite type X, zeolite type A, ZSM-3, EMT, EMC-2, ZSM-18, ZK5, ZSM-5, ZSM-11, xcex2, L, chabazite, offretite, erionite, mordenite, gmelinite, mazzite, and mixtures of these, alumina, silica, carbon molecular sieves, amorphous aluminosilicate and a clay material. Although various cationic forms of zeolites can be chosen, preferably, the adsorbent is a Li containing or Li and bivalent cation containing or Li and trivalent cation containing zeolite of type A and X, which contains Si/Al molar ratio of 0.9 to 1.25, preferably 1.0 to 1.1, and most preferably with Si/Al ratio of less than 1.08.
As shown by thermogravimetric analysis, there are three different regions of weight losses during the activation of monolith. The low temperature weight loss up to 150xc2x0 C. is due to the weakly adsorbed water in zeolite. The weight loss from about 300 to 400xc2x0 C. is due to the decomposition of binder and for some fibers from 500-600xc2x0 C. is due to the decomposition of fiber. The desorption of strongly adsorbed zeolitic water generally occurs above 150xc2x0 C., its loss overlapping with those of the binder and/or fiber weight losses. The shape of the weight loss curve depends on the type of binder used to manufacture the sheet, the temperature, the time and the nature of regeneration gas. The exact activation temperature depends on the nature of regeneration gas used for the activation. The activation should be carried out in such a way that most of the binder and zeolitic water will be removed without significantly damaging the fiber, which is the structural framework for the monolith. As such, the practice of the present invention will remove more than 50% of the binder from a monolith structure.
The temperature of the heated regeneration gas should be high enough to remove or decompose the binder material and activate the adsorbent materials. The removal of the binder material is not absolute as some binder must remain to bind the adsorbent material particles to the support as well as keep the corrugated structure of the monolith, while corrugated sheets are used to provide spaces in the formed monolith. If the regeneration gas contains an oxidizing gas component like oxygen, the binder can also be removed by the reaction with oxygen.
In the adsorption process embodiment of the invention, a component of a gas mixture that is more strongly adsorbed than other components of the gas mixture is separated from the other components by contacting the gas mixture with the adsorbent under conditions which effect adsorption of the strongly adsorbed component. Preferred adsorption processes include PSA, including vacuum swing adsorption (VSA), TSA and combinations of these.
The temperature at which the adsorption step of the adsorption process is carried out depends upon a number of factors, such as the particular gases being separated, the particular adsorbent being used, and the pressure at which the adsorption is carried out. In general, the adsorption step of the process is carried out at a temperature of at least about xe2x88x92190xc2x0 C., preferably at a temperature of at least about xe2x88x9220xc2x0 C., and most preferably at a temperature of at least about 0xc2x0 C. The upper temperature limit at which the adsorption step of the process is carried out is generally about 500xc2x0 C., and the adsorption step is preferably carried out at temperatures not greater than about 70xc2x0 C., and most preferably carried out at temperatures not greater than about 50xc2x0 C.
The adsorption step of the process of the invention can be carried out at any of the usual and well known pressures employed for gas phase temperature swing adsorption and pressure swing adsorption processes. Typically the minimum absolute pressure at which the adsorption step is carried out is generally about 0.7 bara (bar absolute), preferably about 0.8 bara and most preferably about 0.9 bara. The adsorption can be carried out at pressures as high as 50 bara or more, but is preferably carried out at absolute pressures not greater than about 20 bara, and most preferably not greater than about 10 bar.
When the adsorption process is PSA or VSA, the pressure during the regeneration step is reduced, usually to an absolute pressure in the range of about 0.1 to about 5 bara, and preferably to an absolute pressure in the range of about 0.175 to about 2 bara, and most preferably to an absolute pressure in the range of about 0.2 to about 1.1 bara.
As indicated above, the process of the invention can be used to separate any two gases, provided that one of the gases is more strongly adsorbed by the adsorbents of the invention than is the other gas under either conditions of equilibrium or non-equilibrium i. e., in the kinetic regime of a process. The process is particularly suitable for separating nitrogen from oxygen, nitrogen and argon from oxygen, carbon dioxide from air, dinitrogen oxide from air and for the separation of hydrocarbons, for example, the separation of alkenes, such as ethylene, propylene, etc., from alkanes, such as ethane, propane, etc., and the separation of straight-chain hydrocarbons from branched-chain hydrocarbons, e.g., the separation of n-butane from i-butane. Type A zeolites with appropriate cation compositions are particularly suitable for the separation of alkenes from alkanes, n-alkanes from i-alkanes and carbon dioxide from alkanes, alkenes and acetylene. The separation of these gases is preferably carried out at ambient temperature or higher, although the separation of nitrogen, oxygen and argon can be carried out at cryogenic temperatures.
It will be appreciated that it is within the scope of the present invention to utilize conventional equipment to monitor and automatically regulate the flow of gases within the system so that it can be fully automated to run continuously in an efficient manner.